Fisher, A. (2004). The logic of the real arguments. Cambridge: Cambridge University Press
Fisher’s The logic of the real arguments (first published in 1988) is an old-school, clear-cut, plain-spoken informal logic textbook. It shows us how to identify the conclusion, how to identify reasons, it tells us that an argument is a piece of reasoning, and that in order for us to accept it, it must contain a valid inference from true premises. You know – the whole deal.
But the book does contain a chapter which, the author claims, is not very often found in other “do-it-yourself” textbooks of logical analysis (with the exception of Thomas’s Practical Reasoning in Natural Language, we are duly informed). It is about ‘for the argument’s sake…” type of arguments. This, as you will have read between the lines, must have something to do with thought experiments. Fisher does not use the label, but analyses, voila!, Galileo’s experiment on falling objects. Which, by the way, is shown to be unwarranted so stick around.
First of all, let me spill out my prejudiced, not yet fully articulated, ideas about supposition. I am very much in line with Fisher account, and it would be cheating if I would use what he says to legitimize what I think. So, for “separational” purposes, here it is. Fisher acknowledges – out of intuition, I suppose, for nobody gets quoted – that a supposition is not an assertion, it is not presented as true. When I say “Suppose we can fly”, I’m (obviously, intuitively, etc.) not committed to the fact that we could fly.
[Just a short juicy digression: “Suppose we could fly” is a philosophically perverse move. If A says this in a conversation with B, then B is not asked to accept the possibility of a world where we do fly but the possibility of a world where we can fly. In other words, we might still not be able to fly in the “Suppose-we-could-fly” world. In the “Suppose-we-could-fly” world we don’t fly, we can fly. Which is dreadfully strange: why should one suppose a world where we can fly but we don’t?]
AAAanyway. Insofar as we both intuitively sense that supposition is not assertion, we agree. Fisher writes:
If someone begins an argument by saying “Suppose that oxygen does not burn” he is not asserting that oxygen does not burn – he is not presenting it as true.
A mathematician who… is not asserting (telling us) that … but asking us to consider the proposition with a view to drawing out its implications (113, my italicys)
The section in italics is what I want to pursue, and what Fisher leaves aside (and ultimately, in the analysis of Galileo, overlooks). Suppositions are not a different kind of assertion – they are not assertions in which the commitment of the speaker is weaker or absent. I say that from a speech act theoretical perspective and by this I mean: a supposition, insofar as it does not commit the speaker to the truth of the proposition, i.e. it cannot count as an undertaking of that commitment.
Lovely. But what are they? I believe they are directives. As described by Searle (1979, p. 13) directives have a different illocutionary point from that of assertive.
The illocutionary point of these consists in the fact that they are attempts (of varying degrees, and hence, more precisely, they are determinates of the determinable which includes attempting) by the speaker to get the hearer to do something.
And look at the syntax! “Suppose …”, “Let’s assume that…”, “Imagine that…” – are attempts of varying degrees to get the hearer to suppose, to consider etc., briefly, to commit to X “for the sake of the argument”. A command requests a specific act “mop the floor”. A supposition asks for a specific act too “take this as true”. A command has a preferred response “yes, sir”. A supposition has a preferred response “ok, I will”. The felicity conditions of a command can be under doubt, so attempts can be made to justify that they hold: “Because I want you to do it”, “Because I want you to do it and I’m your boss” etc. The felicity conditions of a supposition can be under doubt, so attempts can be made to justify that they hold: “For the sake of the argument”, “Because you have hold this view before”, “Because it derives from your theory”. The speech act which I believe is closest to supposition is proposal; hence, the fitting of the “let’s” idiom. Notice that preparatory conditions hold just the same: it cannot be obvious that B will undertake the commitment to A or that he already did. The following dialogue should seem odd:
B: I think that p!
A: Well, let’s assume p!
The reason it does not sound odd is because it jumps over a few steps which are implicitly taken. So,
B: I think the Sun rotates around the Earth
A: Well, suppose the Sun rotates around the Earth.
If a supposition is a request, the above dialogue should be pragmatically odd. I hope making it a bit more explicit will reveal the oddity:
B: I think the Sun rotates around the Earth
A: Well, (let’s) suppose the Sun does rotate around the Earth.
Who’s us in the let’s? Cannot be B. B is already supposing, i.e. B is already committed to p. But if there is no “let’s”, to whom is the request directed? Notice how well “well” fits in there. And well marks the drawing of conclusions (replace it at will with “well then”, “aha! if this is the case”, “then” etc.) I think the only way we can make sense of it, is if we make it into something like:
A: I don’t think p. (Don’t think non-p either, so far). But you do. Since this is the case, I am ready to accept it myself, and critically examine it. (Wait, why am I talking like Socrates?!). So let me – and therefore, since you are already in the reduction ship – let us undertake the commitment to p.
What I am doing, in saying suppose p – if I want to critically test it in a discussion, at least, but I cannot imagine a situation where a speaker supposes p for some other reason, can you? – I say, <AGREED! I COMMIT ALSO for the sake of the argument>. Remember the preferred response to a request? Acceptance, yes. Now fly away from this subject. Imagine two robbers standing behind a fence debating whether to rob a house or not. They squabble back and forth for a while and at the end the more audacious of them convinces the other one to do it. The last one will say:
B: Ok, let’s do it.
Consider one last example: academic writing is full of let’s and let us. (John Searle, accidentally, is an avid user). I’m not saying let in “let’s” should be interpreted literally – that, because of its idiomatic character. The imperative function arises quite independently of the meaning of “let’s”. Notice that, just as with other indirect acts, some are more built in language then others. Consider how oddly polite would “allow us” and “permit us” would be. Thus, aside from its teacher-in-the-class-sounding connotations, let’s cannot be explained unless this role-play (in which the author allows himself something as if he is a different person) is understood. “Thus, let us suppose” is a note-to-self. If A wants to tackle B’s claim and says “OK, let’s suppose p” it is A (not B) who need to commit to p. And it is worth mentioning that A thereafter commits to that because in the discussion he is in fact precisely on the other side, notice the nice fit of:
“I don’t agree, but OK, I will assume p just as you assert it”
Notice, then, the differences and the similarities between some of the ideas presented here and Fisher’s account. I too think that “Speaker does not undertake p” it is a good description of what happens but the way in which this is achieved in context is more roundabout than Fisher observes. I believe supposition cannot be explained outside a difference of opinion – since “I agree, but OK, I will assume p” is odd.
So now, the parties have agreed on the setting, and the antagonist goes:
A therefore B therefore … therefore X
The protagonist can, of course, object. Aristotle, as we shall see, might have objected to some of the intermediate claims extracted from his theory of motion. (Notice, parenthetically, that this time, although roles are played, the speaker is asserting A, otherwise he cannot make an argument. Fisher overlooks this, as I said earlier).
But if he, the protagonist, doesn’t object, then the antagonist has henceforth established – for the purposes of the discussion – the claim:
If A, then X
If what we’re playing here is theory choice, then A is a (or part of a) theory. It is a theoretical commitment.
After this, as the German would say, tout ira de soi.
If A, then X
But X is not the case.
Therefore A cannot be the case.
Old, overcooked modus tollens. Of course, at this point, we should like to say that the protagonist had better object. But, as it happens, the protagonist is seldom there to slap the antagonist (dialectically, I mean, of ocourse) in the face. So, once arrived at the conditional, it is hard to go back, since no sensible bloke would object to non-X. (Don’t be fooled by the easy-talk. You should not agree that this is what usually happens. It is just my feeling that this is how the strategy is commonly used. If the antagonist overlooked a thing that might make X true, he will make the frontpage of the FailWeek journal. Take Einstein’s epic one in clock-in-the-box.
If UNCERTAINTY PRINCIPLE, then you CANNOT MEASURE THE SYSTEM SO THAT BLABLABLA
But you CAN MEASURE THE SYSTEM SO THAT BLABLABLA
Therefore the UNCERTAINTY PRINCILPE cannot be the case.
As it happened, you could not.)
Oh, geez, where were we? A, right, Fisher’s account of Galileo…
To be continued.
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